Method and assembly for measuring an article&#39;s dimensions

ABSTRACT

The object of the invention is to provide a solution to improve the accuracy of surface and/or profile measurements without any time-consuming manipulations for placing and orienting the object on a measurement table. In addition, this assembly aims at overcoming the drawbacks of known designs. The assembly consists of a stationary, horizontal transparent table (1) for supporting the objects to be measured, at least a video camera (2) and a light source (3) mounted one above the other underneath the table (1), on a bridge (4) movable on two slides (5) for moving the video camera (2) and/or the light source (3).

This invention concerns an assembly and a method to measure thedimensions of an object.

In traditional optical measuring machines, the camera is equipped with alens and a CCD sensor allowing an electronic analysis of the object'soutlines, so asking for the object to be moved under the camera's lens;and risking to move the object on the support, due to vibrations inducedby the support's moving mechanisms. Moreover, it is well known thatlenses give distorted images; and because of that, measuringmachines-cannot work with enough accuracy. All optical lenses show achromatic aberration, which results in that the focal plane and thedimensions of the image change with a light wavelength variation. Untilnow, it has been tried to minimise these effects by correcting of thelenses. Nevertheless, even using corrected lenses it is possible tonotice a residual chromatic aberration, which has a bad impact on theimage quality and lowering the measurements accuracy.

This invention wants to supply a solution to improve the measurementaccuracy, as well as to overcome the disadvantages of knownrealisations.

According to this invention, this aim is reached by means of a method,whose characteristics are the following:

a) one or more objects to be measured are put on a stationarymeasurement table,

b) the object is exposed to light,

c) the shape of the object is recognised taking an electronic snapshotof it by means of a video camera so that to compare the obtained shapewith other ones stored into a computer and to choose the stored imagehaving the same shape,

d) the main axes of the object and their direction are then calculatedto determine a coordinates system related to the position of the object,

e) the outline and/or the surface of the object are then examined bymeans of a probe, whose displacement is driven by the stored image,having previously turned the main axes according to the direction of theones of the object on the table,

the differences between the stored image and the one made of the pointsdetected by the probe are then determined.

The assembly to make use of the method is characterized by a stationaryhorizontal transparent table for supporting the objects to be measuredwith at least a video camera and a light source, one of them beingplaced over and the other one under the said table, mounted on a bridgemovable on two slides, for moving the camera and/or the light source.

The annexed drawings show, as an example, the proposed assembly:

FIG. 1 is a global view of the invention, showing only thecharacteristic elements,

FIG. 2 is a view from above of the bridge supporting two cameras,

FIG. 3 is the same view as of FIG. 2, but with one camera and twolenses, and

FIG. 4 is a partial view of the assembly showing another position forthe light source.

FIG. 1 shows the assembly with a transparent support 1, fixed on abearing arm 11 and holding some objects to be measured 7, one of whichis visible on the drawing. A video camera 2 and a light source 3 arefixed on a bridge 4. The camera 2 is placed over the support 1, whilethe light source is placed under it. Of course, it would be possible aswell to exchange the position between the camera 2 and the light source3. The bridge 4 is mounted movable on two slides 5 X and Y, the controlaxes of which are not showed.

The camera 2, equipped with a lens and a CCD sensor 6 with a narrowfield of view can be freely displaced up and down with respect to thesupport 1, on which one or more objects to be measured 7 can be placedin an undetermined position.

So as to facilitate the interpretation of the measurements, the bridge 4can be provided with two cameras 2A and 2B as in FIG. 2. The firstcamera 2A could have a large field of view allowing to electronicallystore the position of the object(s) to be measured, as well as theirangular displacement. The second camera 2B with a narrow field of viewcan be automatically moved from one object to another and proceed to therequired measurement analyses.

A single camera 2 with two lenses 6, one of them having a large field ofview while the other one has a narrow field of view are showed in FIG.3. The lenses are mounted on a transversal slide 8 placed on the bridge4. According to the different measurement tasks, one of the twodifferent field of view will be used.

FIG. 4 shows another arrangement of the light source 9, which ishorizontally or obliquely mounted but by side of the optical axis of thecamera(s) 2A and 2B. A mirror 10 is used to have the light beam fallinginto the optical axis of the camera.

The aberration problem can be eliminated by filtering the light sourceso as to obtain a narrow band-filtered, so resulting in a nearlymonochromatic light. The central frequency has to be of course chosenaccording to the characteristics of the lens. In practice, the lightspectrum has to be chosen in a range falling around a frequency wherethe lens has the maximum value of chromatic aberration. Moreover, thelens is equipped with a filter having the same optical characteristicsas the light source.

The reasons for the choice of the described frequency are the following:

It is not important to reduce the absolute value of chromaticaberration, but it is necessary to having it remaining constant over thewhole used light spectrum. This condition is best approached at afrequency around which the lens has the maximum value of aberration, andat which the first-order derivative of the curve describing thisaberration is near to zero. The residual aberration can be easilycompensated by means of a calibration of the system, so granting for theminimisation of the error over the whole spectrum of the used lightsource.

Another problem comes out when the machine uses moving means to exploredifferent parts of the object to be measured, due to induced vibrations.This problem is still more important if an optical system with a largemagnification ratio is used to achieve a better resolution.

As the largest vibrations occur during accelerations and decelerations,the problem lies in this phase. The need for accelerations anddecelerations arise from the fact that constant emission light sourcesare used with integrating cameras, and so it is necessary to stop thecamera, as it reaches the required position to obtain a sharp image.

To avoid theses problems the present invention proposes a constantmovement of the camera and a pulsed light source. With this solution,disadvantages in connection with accelerations are eliminated.

Usually, it is necessary to "tell" the machine what kind of object ithas to measure and this is accomplished by means of a singleidentification code, permitting to the machine to load the programrelating to the memorised object. Nevertheless, this method shows twodisadvantages. First of all, a manual intervention is required, withconsequent slow-down of the process. Secondly, it is necessary fordifferent objects of the same shape to be placed always in the sameposition and with the same orientation, because the machine is usuallyprogrammed with an absolute coordinates system and not with anobject-related one; moreover, fixing elements used to keep the object inplace cause another slow-down of the process.

On the contrary, the invention proposes a computer program related toobjects to be measured and a coordinates system related to the objects.So, the operator or a manipulator put the object on the support 1regardless of its position. The large field view camera takes anelectronic snapshot of the object, the computer chooses thecorrespondent program and computes position and orientation of theobject. Then, the computer loads the related measurement program andautomatically computes the coordinates related to the position andorientation of the object.

After having recognised the object, its position and its direction, thecomputer starts the measurement session. Following the programmedoutline, recomputed according to the object position and direction, thecomputer orders the light source to give light pulses at the correctmeasurement positions.

With this method it is obviously possible to recognise more than oneobject shape and the measurement session is not restricted to only onetype of objects at a time, as the objects can arrive as the automaticmachines deliver them. Moreover, the measurement can be carried on at ahigh speed, as manual intervention that could slow down the process isnot required.

I claim:
 1. A method for measuring dimensions of an object, said methodcomprising the steps of:placing the object to be measured on astationary measurement table (1); exposing at least one surface of theobject to light supplied by a light source; detecting an electronicimage snapshot of the object, from a surface opposite the surfaceilluminated by the light source, by means of a first video camera (2)having a first lens with a first field of view; automatically comparingthe electronic image snapshot of the object with image informationstored in a computer; determining a position and an orientation of theobject to be measured; displacing a probe according to a displacementdetermined by the stored image information and the determined positionand orientation of the object; switching on the light source for only ashort duration of light pulses; detecting a further image of the objectto be measured by means of the probe; and measuring any differencesbetween the stored image information and the image information detectedby the probe.
 2. A method according to claim 1, further comprising thesteps of utilizing said first video camera with a second lens, which haswith a second field of view which is relatively narrower than the fieldof view of said first lens, as said probe.
 3. A method according toclaim 1, further comprising the step of utilizing a second video camera,which is different from the first video camera, as said probe.
 4. Amethod according to claim 1, further comprising the step of following anoutline of the object (7), via the first video camera (2), withoutstopping.
 5. A method according to claim 1, further comprising the stepof driving the first video camera (2) only according to pointsdetermined from the stored image information.
 6. A method according toclaim 5, further comprising the step of driving the light source by saidpoints determined from the stored image information to provide saidshort-duration of light pulses.
 7. A method according to claim 5,further comprising the step of utilizing a monochromatic light source,which supplies a monochromatic light beam, as said light source.
 8. Ameasurement assembly apparatus comprising a stationary horizontaltransparent table (1) for supporting an object to be measured, at leastone light source (3) being placed on one side of the table (1), firstand second video cameras (2A, 2B) being mounted on an opposite side ofthe table with respect to the light source (3), a first lens, having arelatively larger field of view, being associated with said first videocamera (2A) and a second lens, having a relatively narrower field ofview to allow high resolution measurements, being associated with saidsecond video camera (2B), the first and second video cameras and thelight source being mounted on a bridge (4) movable on two slides (5),for moving the first and second video cameras (2) and the light source(3), said measurement assembly apparatus being used in combination witha method comprising the steps of:placing the object to be measured on astationary measurement table (1); exposing at least one surface of theobject to light supplied by said light source; detecting an electronicimage snapshot of the object, from a surface opposite the surfaceilluminated by the light source, by means of said first video camera(2); automatically comparing the electronic image snapshot of the objectwith image information stored in a computer; determining a position andan orientation of the object to be measured; displacing a probeaccording to a displacement determined by the stored image informationand the determined position and orientation of the object; switching onthe light source for only a short duration of light pulses; detecting afurther image of the object to be measured by means of the probe; andmeasuring any differences between the stored image information and theimage information detected by the probe.